With a view to improving the rolling resistance and to reducing fuel consumption, modern tires contain rubber compounds which, by way of fillers, contain predominantly electrically non-conducting fillers such as silica, which are used, for example, to create the tread strips. A tire of this type is descried, by way of illustration, in publication EP 0 501 227.
Because of the very high resistivity of these compounds, their use has been accompanied by the development of numerous technical solutions aimed at avoiding the build-up of static electricity and at allowing charge to flow to the ground as the vehicle drives along. The disadvantages associated with the build-up of electrical charge are well known to equipment manufacturers and are as wide ranging as disrupting the operation of the radio fitted in the vehicle, causing the occupant to suffer electric shock as he or she gets out of the vehicle, and accelerated ageing of the tire as a result of the creation of ozone.
Hence, tire manufacturers are seeking to market tires which do not have these disadvantages. U.S. Pat. No. 5,518,055 describes a tire of which the tread strip, made from a non-conducting compound, is coated with a thin layer of conducting compound. This layer is in contact with the side wall compounds, which are themselves also electrically conducting, to allow electrical charge to flow.
Another solution set out in publication EP 0 658 452 is to place an insert in the tread strip. This insert, which extends radially, preferably over the entire circumference of the tire, is made out of an electrically conducting rubber compound and connects the external surface of the tread strip either to one of the crown reinforcing plies, each of these plies being electrically conducting, or to any other part adjacent to the tread strip of the tire and which is sufficiently electrically conducting. Numerous improvements have been made to this principle, according to whether the tread strip comprises one or several layers of conducting or non-conducting materials, and these are set out by way of example in publications EP 0 925 903 or EP 0 963 302.
All these methods are aimed at connecting the external surface of the tread strip to a part of the internal region of the crown of the tire immediately adjacent to it, such as the side wall, a crown reinforcing ply or a carcass reinforcing ply, and which has properties of electrical conductivity.
However, recent developments in tires, again aimed at improving the rolling resistance, have led to more widespread use of weakly electrically conducting compounds, based on silica, in most of the parts of the tire likely to perform mechanical work during the phase in which the said tire is rolling along. Thus, such compounds are used to produce the crown reinforcing plies as well as being used to produce the tread strips.
What is meant by a weakly electrically conducting rubber compound is a material based on rubber and that has a resistivity greater than or equal to 108 Ohms/cm. Likewise, an electrically conducting rubber material is to be understood as meaning a rubber-based material having a resistivity of less than 106 Ohms/cm.